How intracellular parasites invade host cells, and how they contrive not only to escape digestion by lysosomal enzymes but instead to stimulate the host cell to nourish them, are questions that bear on mechanisms of natural and acquired immunity to these infectious agents. These questions are of special interest in relation to parasites such as the leishmanias that infect macrophages, the very archetype of a phagocytic cell, and one of the principal cells concerned in immunity. Since the parasite surface must interact with the host cell membrane during entry of the parasite, and since parasite and host membranes constitute the primary sites of all later interactions during the intracellular multiplication of the parasite, an understanding of the nature and physiology of these membranes is essential in any attempt to understand the phenomena of intracellular parasitism. The leishmanias, a group of parasitic protozoa causing a variety of human diseases widespread in the tropics, provide favorable material for such studies. Accordingly, we propose to investigate the composition and immunogenicity of the surface membranes of amastigote and promastigot forms of Leishmania donovani, as well as the nature of the membranes separating the parasite surface from the host cell cytoplasm. Results of these studies would be applicable to the in vitro assay of cell mediated responses in L. donovani infections and to work on infectivity of other hemoflagellates as Trypanosoma cruzi and African trypanosomes.